Typically, signals are transmitted from a source to the destination through signal traces and interconnections such as through-hole vias. Such interconnections, however, can introduce discontinuities in the transmission path, thus detrimentally affecting signal fidelity. For instance, in certain scenarios, high frequency components of the signals may be reflected multiple times, thus preventing the signals to get through to the receiver. Furthermore, it is well established that through-hole signal vias have capacitance to the reference planes that the through-hole signal vias traverse. In particular, it has been recognized that the greater the number of reference planes a through-hole signal via passes through, the higher the capacitance between the through-hole signal via and the ground layers will be. Yet further, the greater the surface area of the ground layer defined in immediate proximity to the through-hole signal via, the higher the capacitance can be. Generally, several shortcomings are associated with discontinuity capacitance (e.g., limiting signal trace speed, increasing bit error rate, etc.).
Currently, reducing capacitance between the through-hole signal via and the ground layers is achieved by forming blind/buried vias or back drilling. In the first alternative, typically, blind vias are formed by drilling holes through only the necessary reference planes. However, implementing blind/buried vias has certain limitations. For instance, forming blind vias is a very costly process.
Alternatively, back drilling can be used to lower through-hole via capacitance. Once the vias have been formed through all the layers of the PCB, portions of the vias not being used in the transmission paths are physically removed. Similar to blind vias, however, several limitations can be associated with back drilling vias. Yield loss and increase in cost are two of such limitations.